1. Field of the Invention
This invention relates to an optical pickup and an optical disc device for recording and/or reproducing signals on or from an optical disc, such as a compact disc (CD) or a CD-ROM, or a magneto-optical disc or phase-transition optical disc (termed collectively as an optical disc). More particularly, it relates to an optical pickup and an optical disc device configured for reproducing plural types of optical discs having different disc formats.
2. Description of the Related Art
Heretofore, an optical pickup for reproducing an optical disc is made up of a semiconductor laser element, as light emitting means, an objective lens for radiating the light from the semiconductor laser element to the optical disc, a biaxial actuator for holding the objective lens for movement in two directions, a photodetector for detecting the return light from the optical disc and a servo circuit for driving the objective lens of the biaxial actuator in both the focusing and tracking directions in a controlled manner.
With the above optical pickup, a light beam from a light source is radiated onto a signal recording surface of an optical disc, and the return light reflected from the signal recording surface is received by a light receiving surface of the photodetector for detecting recording signals.
For correctly detecting playback signals, it is necessary for the light beam from the light source to form a light spot at a correct position on the signal recording surface of an optical disc. To this end, movement of the objective lens of the optical pickup for condensing the light from the light source on the signal recording surface of the optical disc is controlled on the basis of pre-set servo signals.
The objective lens is servo-controlled by tracking servo of moving the objective lens along the radius of the optical disc with respect to the recording tracks on the optical disc and by focusing servo of moving the objective lens in a direction towards and away from the signal recording surface of the optical disc along the optical axis.
The tracking servo system may be classified into a three-spot method in which the light beam is split into three sub-beams and both side sub-beams, and a phase comparison method in which the phase difference of the return light to a four-segment photodetector is detected, as described in U.S. Pat. No. 4,785,441.
As the focusing servo system, there is known an astigmatic method in which astigmatic aberration of a light beam is detected by a photodetector divided into four segments in the vertical and horizontal directions for detecting the focusing error.
Meanwhile, attempts are being made towards raising the recording density of the optical disc as an auxiliary storage device for a computer and as a package medium for the speech and video information. If the conventional optical disc is termed a first sort of the optical disc, it is envisaged with the second type of the optical disc, proposed for realizing high recording density, to reduce the distance between recording tracks of the optical disc for realizing high recording density.
However, if it is desired to reproduce the first type of the optical disc, as the conventional optical disc, and the second type of the optical disc, having the disc format different form that of the first type of the disc, by the same optical pickup, it may be feared that any sole servo system is not sufficient.
If assumed that the above-mentioned three-spot method is used for tracking servo, the spacing between recording tracks becomes narrower with the second type of the optical disc than with the first type of the optical disc, such that the relative position between the spot of the side beam and the track is changed and hence there arises the risk that the tracking signal cannot be produced by the three-spot method.
Similarly, with the phase comparison method, the risk arises that the tracking error signal produced is not optimum since the spacing between recording tracks differs with different disc formats.